I've never quite understood this argument. Why is having a sick child considered as harming the child. The alternative is not having a child, in which case it can't be harmed, because it doesn't exist. If I had a choice between being born sick, or not being born at all, I'd take my chances with being born sick.

It's a hard life, but at least it's life.

I would love to see if you keep this viewpoint a good 10/15 years down the line when you have fully matured and seen a lot more in life. Of course I am assuming here you are in your early life based on what I personally consider to be a very immature response.
You said yourself if you had the choice you would choose to 'take your chances', well consider for one moment not everyone will feel like you and also please consider that once you are born you have no choice.
I cannot for the life of me understand how anyone can choose to bring a child to life with a very high risk of bringing it into this world knowing it will have health problems throughout its life, more probable very severe problems.

We are not talking about normal situations here, we are not talking about your average couple choosing to have a child and something THEN going wrong, in those circumstances there is nothing that can be done and most would accept that.
We are talking about 2 people having a child KNOWING that child has an extremely high chance of having severe health problems. If you cant get your head around this or are happy to still agree with that happening then there is really nothing more for me to discuss with you.